The invention relates to semiconductor devices and more particularly to a method and apparatus for reducing bias voltage drops within a substrate.
Semiconductor devices which perform various functions are constructed on semiconductor substrates using a variety of techniques. The integrated circuits are generally constructed on the upper, active surface of a substrate or semiconductor wafer. It is common to provide a substrate bias voltage Vbb via a plurality of well plugs, such as P-well plugs. The Vbb bias voltage is typically provided by a voltage regulator or a charge pump. The well plugs are electrically connected with the substrate through respective diffusion regions. The substrate bias voltage Vbb is used to control the threshold voltage Vt of various transistors formed in the substrate and maintain a substantively uniform Vt from transistor to transistor. If the substrate voltage Vbb differs across the area of the substrate due to voltage drops it changes the threshold voltage Vt characteristics of nearby transistors causing the transistors to switch inappropriately.
It is known in the art to maintain a stable substrate bias voltage Vbb over a large area of the substrate by spacing the well plugs close together, however this occupies large substrate real estate. It is also known to use a heavily doped substrate with a lightly doped epitaxial layer to help stabilize the substrate voltage; however such processes are expensive. It would be desirable to have a semiconductor device and method of making the same that cost effectively reduces bias voltage Vbb drop across the substrate, and which also reduces the number of P-well plugs required to supply the bias voltage Vbb over a given substrate area.
The invention provides a conductive layer secured to a backside of a semiconductor substrate to help maintain a more uniform level of bias voltage within the substrate. The substrate has transistors fabricated on its upper, active side and has P-well plugs on the upper, active side that electrically couple Vbb voltage from a Vbb voltage source to the substrate. The conductive layer can be a conductive metallic layer, a conductive paste, a conductive polymeric film, or a conductive metallic film and provides a path for removing unwanted voltage or noise from the substrate to help maintain a uniform Vbb voltage throughout the substrate. As a consequence, a more uniform bias voltage Vbb is provided within the substrate and in particular in the proximity of the transistors and thus the number of P-well plugs used to supply the Vbb voltage can be reduced. The backside conductive layer may optionally be directly connected to a Vbb bias source.
Different materials and methods are disclosed for forming and/or securing the conductive layer to the backside of the substrate. In one exemplary embodiment the conductive layer is a metallic layer, which may optionally extend beyond the backside of the substrate to provide an area for a wire bond connection to the Vbb bias source. In other exemplary embodiments the conductive layer may be formed as a cureable conductive paste, a conductive polymeric film, or a thin conductive metal film.